[AMRadio] Sentnel Radio Co. Musicair Console, Help Needed

[Larry Szendrei]

> I am in the process of restoring a Musicair Console Radio.  Replaed
> several defective capacitors.
> The radio plays now, but after warming up for 2-3 minutes, it fades out,
> no audio.
>
> Turn it off, let it rest a few minutes and turn it on.  It plays and then
> does the same thing again.
>
> Anyone have some advice on what to check?  Can a coupling capacitor do
> this?  AGC tube?
> Or, is it a tube?  All the tubes check out fine on a tube tester. 
> Something warms up and changes.
> Any help will be appreciated.  Thanks,  Ron  100% DisAmerVet
>

I've seen a couple of replies to this, and they both offer good advice.

Here is my 2 cents: I wouldn't jump to conclusions - it could be just
about anything - faulty tubes are easiest to find by substituion, if you
have spares. It could also be a bad capacitor or resistor (the former is
more likely than the latter).

On my bench, I would start by monitoring the plate, screen grid, and
cathode voltages of each tube with a high-input impedance voltmeter,
looking for a change in one of those when the signal fades out. This will
help isolate in which stage of the reciver the problem resides. If you see
more than one tube affected, look at what's common to both stages (for
instance, the R or C of a B+ decoupling network.) A change in one of these
voltages can either be the root cause, or a may be just a symptom, or the
problem. Once you get close, sketch out a schematic (if you don't have
one) of the circuit around where you observe the weirdness, and apply
logic to identify which component(s) failures could explain what you see,
in the way they normally fail (for example, bypass capacitors usually, but
not always, become leaky, and resistors usually, but not always, fail high
or open).

You can also isolate whether the problem is before or after the 1st audio
stage by applying a low-level audio source to the grid of the 1st audio
stage, and see if the output from that sorce also dissapears when the
radio goes dead.

Finally, if you have an LF receiver (such coverage is usually included in
modern ham tranceivers, for example) you can use it as a signal tracer to
detect any signal at the output of the mixer, or at the output of the IF
stages - the Intermediate Frequency of broadcast radios is usually, but
not always, 455 KHz.

The most insiduous (most difficult to find and repair) cause of these
symptoms which I have run across  in my experience has been a bad
resonating capacitor inside an IF transformer (in my SX-28), or the mica
capacitor integrated in the plastic base of the smaller "modern" IF
transformers used from the late 1950s until the early 1970s. One notorious
brand IIRC was "k-tran."

Bringing an old radio back to life is always rewarding; good luck with the
endeavor.

-Larry/NE1S

-- 
Pay a visit to my amateur radio web page at:
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